Transgenic, non-human animals can be used to understand the action of a single gene in the context of the whole animal and the interrelated phenomena of gene activation, expression, and interaction. The technology has also led to the production of models for various diseases in humans and other animals which contributes significantly to an increased understanding of genetic mechanisms and of genes associated with specific diseases.
While smaller animals, such as mice, have proved to be suitable models for certain diseases, their value as animal models for many human diseases is quite limited. Larger transgenic animals are much more suitable than mice for the study of many of the effects and treatments of most human diseases because of their greater similarity to humans in many aspects.
For the past two decades, pigs have been used in biomedical research with increasing frequency as replacements for dog and primates. This is due to the anatomical and physiological similarity to humans. Pigs and human share anatomical and physiological characteristics such as heart size, cardiac output, and coronary blood supply which have made pigs widely used in cardiac surgery, pacemaker studies and heart transplantions. Similarly, pig and humans share features in digestive physiology and pigs are therefore widely used in nutritional studies and subjects in relations to this including lipid metabolism, gastric ulceration, diabetes and alcoholism, Furthermore, porcine models are used for the study of disorders of the skin. Organs of porcine origin are also used in organ transplantation research. However, the pig constitutes an evolutionary clade in relation to humans and rodents.
Many human diseases are hereditary. The inheritance of genetic disorders, abnormalities, or traits is a function of both the type of chromosome on which the abnormal gene resides (autosomal or sex chromosome), and of the trait itself, i.e. whether the trait is dominant or recessive. The trait can be due to a single defective gene from one parent (dominant inheritance) or the trait can arise when two copies of the gene (one from each parent) are defective (recessive inheritance).
Dominant inheritance occurs when an abnormal gene from one parent is capable of causing disease even though the matching gene from the other parent is normal. Accordingly, the abnormal gene dominates the outcome of the gene pair and one copy of the mutant gene is sufficient for expression of the abnormal phenotype.
Several distinct characteristics of autosomal dominant inheritance include: Every affected individual has an affected parent (except in cases of new mutations or incomplete penetrance); males and females are equally likely to inherit the allele and be affected (as the genes are located on autosomes, of which each male and female has two copies); and recurrence risk (the probability that a genetic disorder that is present in a patient will recur in another member of the family) for each child of an affected parent is ½ (as only one copy of a gene is necessary for development of the disease). If one parent is a heterozygote for a particular gene, their offspring will either inherit the gene or they will not, with each outcome equally likely. Accordingly, if an affected individual's siblings are not affected, they do not carry the mutation and cannot therefore pass it on to their own offspring.
As many of these autosomal dominant diseases are deleterious, one would expect that over time they would disappear from the population due to natural selection. However, there are several phenomena, cf. below, that can lead to maintenance of these alleles in the population.
Variable expressivity: the variable severity of a genetic trait. Different individuals with the same mutation will develop different degrees of the disorder due to difference in environment and the modifying effects of other genes. Because of this, a mutation that leads to a relatively mild form of the disease in one individual stands a good chance of being passed on and maintained in the population. The same mutation in another individual may lead to such a severe manifestation that the affected individual is unable to propagate the mutation to the next generation. This demonstrates very well the fact that genetic disease results as combination of genetic and environmental influences.
Late onset: when a disease has an onset later in life, affected individuals may have passed the gene to their offspring before they even knew they carried it themselves. One example of this is Huntington's disease, a late onset neurodegenerative disorder. It is now possible to receive genetic testing for this disorder, a practice that leads to many complex issues for the family undergoing the testing.
High recurrent mutation rate: 85% of cases of achondroplasia, a major cause of dwarfism, are the result of new mutations. Some segments of the genome are subject to higher than normal rates of mutation, which can lead to the maintenance of the disease in the population even if both parents were normal. This is particularly true of diseases that affect fertility. If the disease is invariably lethal at a young age, before reproduction is possible, the only source of the disease would be new mutations.
Incomplete penetrance: phenomena where a portion of individuals with a disease-associated genotype do not develop a disease. If only 30 people out of 50 who have a disease-associated mutation actually develop the disease, it is incompletely penetrant. A disease that is 75% penetrant is one in which 75% of those who carry the disease-associated mutation eventually develop the disease. The rest do not.
Transgenic animals carrying a dominant disease gene which is expressed in the animal makes it possible to study the phenotype associated with said dominant disease gene if the gene when expressed in the animal actually leads to the same disease as in humans. Transgenic animals have traditionally been used for the improvement of livestock, and for the large scale production of biologically active pharmaceuticals. Historically, transgenic animals have been produced almost exclusively by microinjection of the fertilized egg. The pronuclei of fertilized eggs are microinjected in vitro with foreign, i.e., xenogeneic or allogeneic DNA or hybrid DNA molecules. The microinjected fertilized eggs can then be transferred to the genital tract of a pseudopregnant female.
Only a few examples of success with sperm-mediated gene transfer methods in monkeys and mice have been reported (reviewed e.g. by Vodicka (2005): Ann. N.Y. Acad. Sci.; 1049: 161-171; Chan (2004): Reprod. Biol. Endocrinol.; 2:39; and by Wall (2002): Theriogenology; 57: 189-201).
As noted by Wall (ibid), only few studies convincingly demonstrate transgene expression. Wall (ibid) concludes that the body of evidence is still not sufficient to warrant elevating sperm-mediated gene transfer to the status of other available state of the art methods.
Smith (2004): Int. J. Med. Sci.; 1(2):76-91; notes that sperm-mediated gene transfer has not yet become established as a reliable form of genetic modification and that concerted attempts to utilise sperm-mediated gene transfer often have produced negative results.
WO 2005/038001 is directed to a method for producing transgenic animals.
US 2005/0053910 pertains to cell culture media for sperm-mediated gene transfer methods.
JP 2000-316420 is related to transgene pigs obtained by methods involving micro-injection and not sperm-mediated gene transfer. The pig may carry a gene causing an autosomal, dominant disease.
However, as pigs constitute a distinct evolutionary clade in comparison with humans the introduction of mutations known as disease causing mutations in specific genes in humans cannot be expected to yield a desired phenotype in the pig model.
There is a need for improved animal models for human diseases in order to gain more information of the onset, progression and treatment regimes of hereditary diseases in humans.